Field of the Invention
The present invention relates to the generation and processing of extreme ultraviolet radiation. It refers to an optical collector for collecting extreme ultraviolet radiation according to the preamble of claim 1. It further refers to a method for operating such an optical collector, and a EUV source with such a collector.
Discussion of Related Art
Extreme ultraviolet radiation (EUV) is emitted by hot dense plasmas which can be produced by exciting a target material e.g. tin, with a focused laser beam, creating a laser produced plasma (LPP). A part of the radiation emitted from this plasma is in the EUV spectrum of wavelengths between 10 and 100 nm. The major share of emitted energy lies outside this wavelength band, comprising ultraviolet, visible, infrared and reflected laser radiation. To achieve a high power output and a high brilliance of the radiation source, the emitted radiation is collected and collimated to an intermediate focus for further usage. This is done by ellipsoidal collector optics.
FIG. 7 shows a simplified configuration of a EUV source. The EUV source 10 comprises a chamber 11 containing an elliptic or nearly elliptic multilayer (Mo/Si) collector or mirror 15 and a target delivery system 17, which is attached to the chamber 11 by means of a mechanical support 16 and emits a chain of droplets 19 of the target material. A high power (100 W to 20 kW) and high repetition rate (10 Hz to 500 kHz) drive laser 12 ignites the target material at a EUV production site 20. The focused drive laser pulse 14 enters the chamber 11 through a flanged window 13. The spatial and temporal characteristics of the laser pulse match the target size and location in order to maximize conversion efficiency (CE), i.e. the ratio of EUV energy and laser energy. An optical system 23 is used to detect and control the droplets 19 coming from the target delivery system 17.
The collector 15 collects the EUV light 18 generated at the EUV production site 20. The collector 15 has a first focus at the EUV production site 20, and a second focus 21, called intermediate focus (IF), where the EUV light 18 is bundled for further use in a subsequent EUV lithography tool (not shown in FIG. 7). The collector 15 has an aperture 22 for the laser light to reach the EUV production site 20.
The EUV target delivery system 17 delivers the plasma source material to the EUV production or ignition site 20. The source material is in the form of liquid droplets 19 of either pure material, e.g. Sn, Xe or Li, or of a suspension of target material in a solution, e.g. water or alcohol. The delivery of the droplets 19 of source material takes place at a constant repetition rate and droplet or target size. Target sizes are in the range of 10-100 pm in order to minimize the amount of neutral particles being present after the plasma formation. As has been mentioned before, the targets or droplets 19 reach the EUV production site 20 at the first focal point of the EUV collector 15. Similar configurations are shown in documents WO 2006/091948(A1) or WO 2009/025557(A1) or WO 2010/017892(A1).
The out of band emissions which are partially absorbed in the reflective optics lead to increased temperatures of the collector surface. To avoid thermally induced deformations and a deterioration of the multilayer coating, the collector 15 has to be cooled. However, any gas absorbs the EUV radiation and therefore the radiation sources and collimating optics are operated in a vacuum. This prohibits convection cooling of the collector surface within the chamber 11. Therefore cooling has to be implemented in another way.
For a normal incidence collector the radiation hitting the collector surface is not homogeneous. Due to directionally varying emissions and varying distance between the collector surface and the plasma, there are regions of the collector surface with higher heat load than others, which results in temperature gradients across the surface. Both elevated temperature level and temperature gradients induce thermal stresses which lead to collector deformation.
Deformations of the collector surface can be reduced by a rigid design of the collector surface. The choice of material also has a strong influence on the deformations in operation. Mechanical forces on the reflective part of the collector can induce or compensate for deformations.
Document JP 8211211 proposes a design for high power laser optics, which are cooled from the back side. To avoid deformations of the reflective part by the pressure of the coolant the mirror is designed stiffer than the cooling ducts which mitigates all pressure induced deformations to the back structure.
Document DE 19955574(A1) describes a gas cooled reflector for high power laser radiation. The design is such, that the thickness of the reflector substrate is reduced to a minimum (e.g. 1 to 25 mm) to enhance convective cooling of the mirror without losing the required stiffness to prevent vibrations or deformations. Further a cooling scheme based on convection on the collector back side is proposed. Ribs, which are designed on the collector back side to enhance manufacturability, serve for cooling enhancement by surface extension and flow perturbation as a secondary benefit.
Documents US 2007058244(A1), US 2009289205(A1) and EP 2034490(A1) disclose normal incidence EUV collector designs and reflector arrangements, without any reference to thermal management, cooling or deformation control of the proposed optics.
Document U.S. Pat. No. 7,641,340(B1) describes a cooling setup for optical surfaces in near vacuum based on heat transfer through a liquid in a narrow gap between the back side of the optics surface and a temperature controlled member. This heat transfer is based on conduction and the liquid is kept in position by interfacial surface tension.